The present invention relates to semiconductor devices responsive to ions in general and is concerned more particularly with devices comprising a field effect transistor.
New devices have already been developed for detecting and measuring chemical properties, such as the activity of ions, of a liquid medium. Among these known devices is the ISFET transistor (Ion-Sensitive Field Effect Transistor) the structure and operation of which are described, for example, in U.S. Pat. No. 4,020,830. The ISFET transistor is a field effect transistor with an isolated gate, the gate of which is replaced with a conducting solution and the insulating layer of which reacts in a specific manner with certain ions in the solution. The response of the ISFET transistor is therefore dependent on the nature and concentration of the solution. One of the problems that exists with the known structures is to avoid interference between the solution and the purely electronic elements, namely the channel, source and drain regions, and the connections of the device. The present materials and techniques of encapsulation do not enable this disadvantage to be completely eliminated. Moreover, the responsive layer which is in contact with the solution and with the channel zone should be relatively thin, typically of the order of 0.1 micron, in order to ensure good operation of the transistor. Now, this responsive layer should also serve as a barrier to the migration of ions of the solution towards the electronic device, and it will be easily understood that this barrier becomes much less effective as its thickness is reduced.
One known method for avoiding interference between the solution and the drain and source contacts consists in providing deep diffusions across the substrate and making the drain and source connections on the back of the silicon chip. This method obtains the advantage of eliminating any connection zone on the surface which is in contact with the solution. However, the formation of these deep diffusions across the substrate is not compatible with the standard technological procedures and these diffusions are, moreover, fairly badly controlled.